The minimization of cardiac pacer battery drain is a goal toward which considerable effort has been expended. For example, a major benefit of demand cardiac pacers, as opposed to asynchronous pacers, is their significant potential energy savings. Such devices sense natural heartbeats to inhibit stimulating pulses to the heart in the presence of natural heartbeats, and provide periodic stimulating pulses to the heart in the absence of natural beats. Inasmuch as the greatest battery drain results from the generation of stimulating pulses, the inhibiting of these pulses significantly reduces battery drain.
Typical prior art demand devices inherently carry high safety margins. The stimulation energy is set at an amount known to be several times that of the maximum required threshold energy in most patients. As the threshold energy of a patient is not constant, and may vary during the day, and as the threshold information is not known to the device, energy well above the expected threshold, and often maximum energy, is provided for stimulation. Obviously, this results in a high drain from the power sources and the result is shortened life time for the cardiac demand device.
A device which ameliorates the problem of high power source drain is disclosed in U.S. Pat. No. 3,757,792 of Pieter M. J. Mulier and John R. Helland issued Sept. 11, 1973 for Automatic Threshold Compensating Demand Pacemaker, which is commonly owned with the present invention. This prior art device provides for greatly decreased battery drain and thus greatly increased device lifetime by sensing each driven heartbeat and providing for a decrease in energy of each succeeding stimulating pulse until such time as no driven heartbeat occurs. When loss of capture is sensed, the next succeeding stimulating pulse is increased in energy by an amount to be safely over the threshold hysteresis level. An embodiment is also disclosed which provides an earlier extra stimulating pulse of maximum energy following loss of capture, with the following normal pulse reduced to a level only sufficiently above the threshold hysteresis level to be safe. Further stimulating pulse energy reductions occur from this reduced pulse until capture is once again lost.
By operating as described, the device of U.S. Pat. No. 3,757,792 provides for greatly decreased battery drain and thus greatly increased device lifetime. However, by decreasing the energy level of each succeeding pulse from an energy reduced pulse which itself is just above threshold, this prior art device results in frequent loss of heartbeat or, in the alternative embodiment, in frequent maximum energy pulses. For example, in the operational diagrams illustrated in FIGS. 4 and 5, every fifth stimulating pulse is below threshold which results in either a loss of heartbeat or a maximum energy stimulating pulse. Such frequent losses of heartbeat are, at the very least, disconcerting to the patient while maximum energy pulses at that same occurrence rate offset at least some of the power drain savings attending the device.